Known in the art is an internal combustion engine arranging an NOx selective reduction catalyst in an engine exhaust passage, arranging a urea aqueous solution feed valve in the engine exhaust passage upstream of the NOx selective reduction catalyst, and using the ammonia produced from the urea aqueous solution fed from the urea aqueous solution feed valve to selectively reduce the NOx contained in the exhaust gas, the internal combustion engine arranging NOx sensors at the inlet and outlet of the NOx selective reduction catalyst to find the NOx reduction rate by the NOx selective reduction catalyst from the output values of these NOx sensors, judging that the NOx selective reduction catalyst is abnormal when the NOx reduction rate found from the output values of the NOx sensors falls below a predetermined NOx reduction rate range, and judging that the NOx sensors are abnormal when the NOx reduction rate found from the output values of the NOx sensors exceeds a predetermined NOx reduction rate range (for example, see Japanese Patent Publication (A) No. 2006-37770).
However, with an exhaust purification system using an urea aqueous solution, sometimes the NOx purification rate drops due to clogging of the urea aqueous solution feed valve and other problems, a drop in concentration of the urea aqueous solution, illicit use of liquids other than an urea aqueous solution, etc. In such a case, with the above-mentioned internal combustion engine, there is a problem that it is not possible to pinpoint the reason for the drop in the NOx purification rate.